David H. Krinsley

Mjølnir structure: An impact crater in the Barents Sea

A systematic search for impact indicators was conducted on a core of Late Jurassic-Early Cretaceous sedimentary strata from the vicinity of the proposed Mjolnir impact structure, Barents Sea. A 0.8 m-thick section of the core was found to contain unequivocal indicators of meteoritic impact: shocked quartz grains and a strong enrichment in iridium. The ejecta-bearing strata were discovered only 30 km north-northeast of the structure, within a stratigraphic interval corresponding to the seismically defined deformation event at Mjolnir. Further study of this unusually well presented impact-crater-ejecta-layer pair may help constrain poorly understood aspects of large-magnitude meteorite impacts into the oceans. 14 refs., 4 figs.

Loess deposits associated with deserts

Summary The division of loess deposits into warm/desert and cold/glacial is well entrenched in the literature. The nature and distribution of glacial loess are well accounted for but doubts have been cast on the existence of a true desert loess, i.e. a loess consisting of silt particles actually formed in a desert region. It appears that the loess deposits in Central Asia and North China were formed from silt particles transported out of adjacent deserts — but the particles themselves had their origins in glacial grinding and cold weathering processes in the adjacent mountains. Thus the deserts close to mountainous areas tend to have associated loess deposits. Deserts which are not particularly associated with mountains, e.g. the Sahara and Australian deserts, do not have large loess deposits nearby. Some loess deposits in the Persian/Arabian Gulf region have been observed which consist of silt-sized carbonate particles; a separate division of carbonate loess is proposed. Large scale silt formation is still one of the consequences of glacial action, however it is apparent from recent studies of quartz clasts that glacial grinding is not such an exclusive quartz silt producer as was once proposed. Much coarse-silt-sized quartz is produced directly by weathering of igneous rocks, but there appears to be little doubt that the vast majority of the silt particles for the loess deposits of the world were formed as a consequence of glacial action and cold weathering — and this includes the deposits in Central Asia and North China.

Provenance interpretation of quartz by scanning electron microscope–cathodoluminescence fabric analysis

We used a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM) to study patterns of variable-intensity CL in quartz grains from a variety of igneous, metamorphic, sedimentary, and shock-deformed (meteorite-impact) rocks. Distinctive fabrics in quartz grains revealed by SEM-imaged differential CL include zoning, healed fractures, complex shears, planar features (shocked quartz), dark CL streaks and patches, indistinct, mottled texture, and nondifferential (low-contrast) CL. Zoning is common in volcanic quartz and some plutonic quartz. Zoned plutonic quartz is distinguished from volcanic quartz by the presence of closed fractures and dark CL streaks and patches. Metamorphic quartz displays either an indistinct, mottled texture, or nearly uniform (nondifferential) CL. Quartz from rocks severely deformed by tectonism displays a complex pattern of multiple, small-scale shears. Quartz from meteorite-impact sites and some system boundaries is characterized by intricate patterns of planar features, presumably created by shock metamorphism. Thus, the SEM-CL fabric-analysis technique provides a rapid method for distinguishing quartz from a variety of source rocks.

Is Quartz Cathodoluminescence Color a Reliable Provenance Tool? A Quantitative Examination

ABSTRACT We examined cathodoluminescence (CL) colors of quartz by using red (590-780 nm), green (515-590 nm), and blue (380-515 nm) optical filters interfaced with a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM). SEM/CL images taken through these filters were captured digitally and transferred to a computer. Luminescence intensities (luminosities) of the images were measured by using available commercial software. Measured luminosities of these CL images are directly related to relative intensities of red, green, and blue CL emissions. Luminosity data were then used to construct plots that display relative luminosities of the CL images acquired through the red, green, and blue filters. An unfiltered CL image of each quartz grain, generated by photons with wavelengths ranging from 200-700 nm, was also acquired. By subtracting the numerical luminosity values of the images acquired through the red, green, and blue filters from the luminosity value of the unfiltered image, the contribution to total luminosity provided by CL emission in the near ultraviolet (UV) was calculated. The CL colors of quartz from a variety of volcanic, plutonic, and metamorphic rocks and hydrothermal deposits were examined. Volcanic quartz phenocrysts have the most restricted CL color range, with strongest emission intensity in the blue wavelength band. CL colors of plutonic quartz overlap those of volcanic phenocrysts but extend over a broader range to include quartz that displays higher intensity of red emission. CL emission in hydrothermal (vein) quartz is similar to that in plutonic quartz, although some hydrothermal quartz exhibits stronger green-CL emission than does plutonic quartz. The CL colors of metamorphic quartz exhibit the widest variation, overlapping the color fields of both volcanic and plutonic quartz and extending further into the red. CL emission in the near UV makes a significant contribution ( 5-85 percent) to the total luminosity of SEM/CL images, particularly images of plutonic quartz. Because of overlap in the CL color ranges of volcanic, plutonic, metamorphic, and hydrothermal quartz, unambiguous identification of quartz provenance on the basis of CL color alone is problematic. It is difficult to distinguish between volcanic and some plutonic quartz, and between some plutonic and hydrothermal quartz, or to distinguish magmatic quartz from metamorphic quartz that exhibits blue CL color. Only metamorphic quartz that exhibits moderately strong red emission appears distinguishable (on the basis of color) from quartz of other origins. Our work thus suggests that CL color is not a reliable indicator of quartz provenance.

FACTORS THAT INTERFERE WITH THE AGE DETERMINATION OF ROCK VARNISH

Over the past few years the rock varnish literature has proliferated. More than a dozen research groups in at least six countries are now investigating the potential of rock varnish as a Quaternary dating technique for geomorphical and archaeological studies. New backscattered electron microscopy imagery of varnishes from southwestern North America illustrates the need for great caution in the selection of samples for all varnish dating methods. Nine general factors that may interfere with varnish dating are presented. For varnish dating to be successful, these factors must be evaluated by careful examination of samples in the field and by analyses of many cross-sections in the laboratory. [Key words: rock varnish, Quarternary dating methods, geomorphology, backscattered electron microscopy, geochronology.]

Application of backscattered electron microscopy in shale petrology

A grey pyritic mudstone from Central Wales (Red Vein unit of the Dicellograptus anceps zone, Upper Ordovician Ashgill) has been examined in thin section by scanning electron microscopy using backscattered electrons. Using backscatter it is possible to identify individual mineral constituents of the mudstone by virtue of their atomic number (Z) contrast and differential hardness (relief). The amount of detail observable is far greater than that possible with optical microscopy. Valuable information can be obtained relating to particle form, orientation, texture and internal structure which aids in interpretation of the deformational and diagenetic history of the rock. The adoption of electron microscopical methods in the study of thin sections and polished rock chip surfaces promises to revolutionize the field of shale petrology.

Nanometer-Scale Layering in Rock Varnish: Implications for Genesis and Paleoenvironmental Interpretation

Manganiferous rock varnish collected from Death Valley and Antarctica contains the smallest known terrestrial sedimentary deposits, with some layers only a few nanometers thick. Irregularities in these nanometer-scale layers are consistent with shrinking, cracking, and weathering of clay minerals. In the Death Valley rock varnish, very different High Resolution Transmission Electron Microscope (HRTEM) textures coexist that may be related to climatic change. HRTEM observations contradict previous microbial models of Mn-Fe enhancement, requiring a new three-step model of biomineralization and diagenesis for varnish formation.

Cation-leaching sites in rock varnish

Backscatter electron microscopy of subaerial rock varnish reveals porous textures that are sites of cation leaching with lowered (K + + Ca 2+ )/Ti 4+ ratios and concentrations of manganese and iron. Laboratory studies show that rates of potassium and calcium leaching increase with temperature,time,and surface area, and decrease when varnish interdigitates with coatings of amorphous silica. Only varnishes with continuous layering should be used in paleoenvironmental research and paleomagnetic studies, not textures indicative of postdepositional modifications.

Manganese-rich rock varnish does occur in Antarctica

Abstract Despite accounts to the contrary, we have found that Mn-rich rock varnish is present in Antarctica, as others have before. It is chemically and texturally similar to many varnishes found in lower latitudes. Antarctic varnishes show considerable potential as a dating and paleoenvironmental research tool.

Eolian sedimentation on Earth and Mars: Some comparisons

Abstract Eolian sediments on Earth are mostly formed from quartz; they consist, in large part, of eolian sand deposits in deserts, silt and loess deposits in and adjoining present and former glaciated areas, and finally clay-sized particles carried in suspension for relatively long distances and deposited in oceanic areas by winds. The quartz particles in these regimes originally came from a granitic source; stresses in granitic rock formation, glacial action, and wind abrasion are largely responsible for making the particles available for the three kinds of eolian deposits. With respect to eolian sediments on Mars, it appears that an entirely different set of criteria must apply, but some critical parameters can usefully be compared. Evidence for free quartz on Mars is lacking and sand-sized particles are probably basaltic, although there does appear to be a deficit in the sand size range. Glacial action does not appear to be available as a large-scale particle producer but high-velocity winds could be efficient producers of very fine particles. Fine particles may aggregate in a similar way to that observed in the Australian regions where “parna” is seen; this could supply a silt mode on Mars. Impact experiments with basalt in eolian abrasion devices suggest that basalt sand-sized particles fragment rapidly to produce silt and clay-sized detritus. Cohesive forces must be more effective on Mars since the gravitational contribution to the bond/weight ratio ( R ) is lowe; if R = 1 at about 100 μm on Earth, then R = 1 at about 140 μm on Mars and a much greater range of deposits will be stable. Compared to the terrestrial situation, both larger and smaller particles can be expected to make significant contributions to eolian sediments on Mars. The low gravity and the high speed of moving particles and the relatively weak rock material of which they are composed will allow large-scale fine particle production.